In general, in the manufacturing process of a semiconductor integrated circuit, forming a thin film of a metal or a metal compound such as W (tungsten), WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride), TiSi (titanium silicide), Cu (copper), Ta2O5 (tantalum oxide) or the like has been performed to form a wiring pattern on a semiconductor wafer surface or to fill up a recess formed between wirings or a contact recess. Among these thin films, a tungsten film is widely employed because it has a small resistivity, a low film deposition temperature, and so on. By reducing WF6 (tungsten hexafluoride) employed as a source gas by using, e.g., hydrogen, silane, dichlorosilane or the like, tungsten is deposited to thereby form the tungsten film.
In case of forming the tungsten film, the tungsten film is deposited on a barrier layer formed on the wafer surface in advance in order to enhance its adhesivity and to suppress its reaction with a silicon underlayer. A Ti film, a TiN film or a laminated film thereof is thin and uniformly formed on the wafer surface as the barrier layer. In order to form the tungsten film, WF6 (tungsten hexafluoride) gas and H2 gas are generally used as film forming gases due to their high film forming rate. However, depositing the tungsten film directly on the barrier layer by employing the deposition gases gives rise to problems as follows. First, there may be a long incubation period in which there occurs no film deposition even while the film forming gases are supplied, thereby deteriorating a throughput of processing. Further, fluorine in the WF6 gas may be diffused into the barrier layer to react with the Ti (titanium) such that the reacted portion of the barrier layer swells up in a protruded shape, thereby resulting in defects in an integrated circuit device.
To solve the problems, e.g., Japanese Patent Laid-Open Application No. 2003-193233 discloses a film forming method as follows. Namely, before depositing a film by using WF6 gas and H2 gas, a very thin layer of a seed film made of crystal nuclei of tungsten is formed by using the WF6 gas and a gas having a higher reducibility than the H2 gas, e.g., a silane-based gas such as monosilane (SiH4) or the like. Thereafter, the WF6 gas and the H2 gas are supplied to deposit a film on the top of the seed film by CVD (Chemical Vapor Deposition), thereby forming a main tungsten film.
The above film forming method will be briefly described with reference to FIGS. 10A and 10B. A semiconductor wafer W, which is a target object, is mounted on a mounting table 2. The mounting table 2 is installed in an evacuable processing chamber (not shown). By applying a clamp ring 4 to make a contact with a peripheral portion of the wafer W to press the wafer W against the mounting table 2, the wafer W is kept to be prevented from being slid to side for example. In such a state, the WF6 gas, the SiH4 gas and the H2 gas serving as the film forming gases are supplied to deposit a seed film 6 (first thin film) made of crystal nuclei of tungsten on the wafer W as depicted in FIG. 10A. Then, as shown in FIG. 10B, the WF6 and the H2 gas are simultaneously supplied as the film forming gases to thereby form a main film 8 (second thin film) made of metal tungsten starting from the top of the seed film 6 at a higher film forming rate as shown in FIG. 10B. Further, in the serial film forming processes, an inert gas, e.g., Ar is supplied as a backside gas to a backside of the mounting table 2 in order to prevent the film forming gases from turning around into the backside of the mounting table 2.
As described above, the main film 8 is formed by depositing the thin film starting from the top of the seed film 6, thereby enhancing the overall film forming rate. Furthermore, since the seed film 6 functions as a barrier, fluorine in the WF6 gas can be prevented from being diffused into an underlayer of a TiN film while the main film 8 is deposited.
However, even though the backside gas is supplied to the backside of a mounting table 2 when the main film 8 shown in FIG. 10B is deposited, the film forming gases infiltrate deeply into innermost region of the gap 10 between a lower surface of a clamp ring 4 and a top surface of a peripheral portion of the wafer W. This is because, e.g., a process pressure at this time is set to be higher than that of forming the seed film 6 shown in FIG. 10A. Accordingly, as shown in FIG. 10B, the main film 8 is formed such that an outer edge 8A of the main film 8 entirely covers an outer edge 6A of the seed film 6 and is further enlarged to an outer side of the outer edge 6A. As a consequence, a peripheral portion 12 of the wafer at the outer side of the outer edge 6A of the seed film 6 is exposed to and attacked by an excessive amount of fluorine in the WF6 gas. Further, since the main film 8 is directly formed on the peripheral portion 12 of the wafer not through the seed film 6, fluorine is diffused into the barrier underlayer to react with Ti therein, resulting in the reacted portion thereof being swollen up in a protruded shape.